Uploading an attachment fails with "AssertionError: write() before start_response()" (mod_fcgid)

[arroz]

Running Trac 1.5.4.dev0 (0182158c5cb676af5b40cf1714c3d29c28a9d632 from the git repository). Not sure if the problem actually happens in 1.5.3, I need to be on trunk due to a different issue, so consider this 1.5dev (that version isn't available in the list).

When uploading an attachment to a ticket, I get the following error on the browser. This doesn't seem to depend on either the name or size of the attachment (a one byte file with a name with no special chars or spaces triggers the problem).

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/trac/web/_fcgi.py", line 572, in run
    protocolStatus, appStatus = self.server.handler(self)
  File "/usr/local/lib/python3.9/site-packages/trac/web/_fcgi.py", line 1250, in handler
    write(b'') # in case body was empty
  File "/usr/local/lib/python3.9/site-packages/trac/web/_fcgi.py", line 1203, in write
    assert headers_set, 'write() before start_response()'
AssertionError: write() before start_response()